This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. Unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Semiconducting materials, in particular, wafers made of Si and other III-V materials that are widely used in the semiconductor industry, can strongly interact with electromagnetic fields in the ultraviolet (UV)-visible-infrared (IR) regimes due to their unique band structures. Nanostructures made of noble metals, such as gold and silver, also exhibit strong responses to electromagnetic fields in specific spectral region because of surface plasmon resonance (SPR) that is induced by the resonant oscillation of surface conduction electrons in the surfaces of the illuminated metal nanostructures. Therefore, metal nanostructure/semiconductor composites generated through directly depositing metal nanostructures on semiconductor substrates represent a promising class of functional materials.
Such materials have unique optical, electronic, optoelectronic, magnetic, and catalytic properties. Accordingly, the materials have applications in energy related areas, for instance, photovoltaic cells, photoelectrochemical splitting of water, for generating hydrogen, photocatalysis, and others. For example, single-crystalline semiconductor wafers covered discontinuously with nanostructures of noble metals are applicable to serve as photoelectrodes of high-performance photoelectrochemical cells (PECs) that are efficient for solar energy conversion into electricity and to storing chemical fuels, such as hydrogen. PECs have a number of advantages in comparison with traditional solid-state photovoltaic devices; for instance, PECs do not require high purity and precisely controlled doping levels (in terms of dopant type, concentration, and junctions) of the semiconductor wafers, which mainly determine the fabrication cost of solid-state photovoltaic cells. In addition, PECs built with single-crystalline semiconductor wafers exhibit fast charge transportation, resulting in the elimination of electron-hole recombination and an enhancement in performance, due to high charge mobilities in single-crystalline semiconductors.
However, a long-term challenge in assembling high-performance, stable PECs with semiconductor wafers is to prevent corrosion of the semiconductor photoelectrodes caused by the holes generated in the photon-induced charge separation processes. For instance, n-type Si wafers modified with Pt nanoparticles have shown to increase the performance (e.g., increase of open circuit voltage and output current) of PECs cells fabricated with the Pt/Si composite electrodes as well as enhance the stability of the photoelectrodes. Although greater coverage of the substrate with Pt nanoparticles enables higher stability, the light absorption efficiency decreases due to the blockage of light. Deposition of a submonolayer of well-separated nanoclusters made of noble metals on the surface of a semiconductor wafer could also significantly increase the stability of the PEC fabricated with the metal/semiconductor hybrid photoelectrode. The ideal morphology of each metal nanocluster is a “mushroom” shape, which has a thin root with a diameter of less than about 10 nm bonding to the semiconductor substrate and a large cap with suitable geometry and size for exposing the semiconductor surface as much as possible in order to harvest sunlight.
Metal nanostructure/semiconductor composites are also applicable in controlling quantum yields of semiconductor quantum dots. For example, quantum dots can be tuned in either quenching or enhancing mode by locating metal nanoparticles in the vicinity of the quantum dots with various spacings. Titanium dioxide (TiO2) nanoparticles (a class of strong absorbers of UV irradiation) decorated with Au nanoparticles exhibit enhanced efficiency to convert solar energy into electricity because the SPR states of Au nanoparticles in the visible region can be strongly excited by solar irradiation and the excited electrons can be injected into the conduction band of TiO2. The charge flow drives the hybrid system to behave as a photovoltaic device similar to a dye-sensitized solar cell.
Still further, controlling the hydrophobicity of solid surfaces has a wide range of applications, including anti-sticking, anti-contamination, self-cleaning, and oil/water separation. In particular, it is desirable to create superhydrophobic surfaces that exhibit apparent contact angles larger than 150°. Various techniques are known to enhance the hydrophobicity of a surface by coating the surface with thin layers of low surface energy materials (for example long-chain alkyl thiol molecules for precious metals, or fluoroalkylsilane molecules for Si). However, achieving superhydrophobicity through this strategy can often be difficult. Various physical approaches including lithographic patterning and etching, molding, and imprinting have also been used to create roughness on surfaces to achieve superhydrophobicity. Still other approaches induce surface roughness by direct deposition of micro/nanostructures of different materials (e.g., metals, polymers, polyelectrolytes, oxides, or carbon nanotubes) on substrates through chemical reactions and/or assembly processes have been shown to lead to superhydrophobicity. Nevertheless, producing composite surfaces with a range of superhydrophobic behavior through relatively simple approaches remains difficult.
Properties and performance of metal nanostructure/semiconductor hybrids are strongly dependent on the properties of the metal nanostructures, which are sensitive to their morphologies. A number of solution-phase synthetic approaches have been demonstrated to be capable of producing nanostructures with various well-defined shapes. These approaches have been developed to successfully synthesize metal nanostructures with a large number of well-defined shapes such as spheres, cubes, tetrahedra, octahedra, rings/frames, rods/wires/beams, stars, plates, boxes, and cages. However, synthesis of these structures requires uses of surfactants. Some of these approaches have been extended to grow metal nanostructures with well-defined shapes on solid substrates. For example, seed mediated synthetic processes with assistance of surfactants can grow gold nanorods on silicon or glass substrates and gold nanoplates on conductive indium tin oxide substrates. The as-synthesized shaped metal nanostructures can be deposited on appropriate semiconductor surfaces by controlling surface chemistries and assembly strategies. In particular, the solution-phase chemical reactions have also achieved success in preparing metal nanoplates with the assistance of specific surfactant molecules, for instance, polymeric chains (e.g., poly(vinyl pyrrolidone) or PVP, polyamine), micellar assemblies (e.g., cetyltrimethylammonium bromide or CTAB, di(2-ethyl-hexyl) sulfosuccinate or AOT), coordinating ligands, biological reagents, etc.
The various conventional methods typically use surfactant molecules that are usually employed in the solution-phase reactions to assist the anisotropic or isotropic growth of metal nanoparticles as well as prevent the nanoparticles from aggregation and sintering. However, the surfactants can deleteriously influence the metal/semiconductor interfaces for applications. The use of surfactant molecules also complicates the reactions and contaminates the surfaces of the resultant metal nanostructures, leading to degradation of their performance in certain applications. For instance, a thick layer of organic surfactant molecules on the surfaces of metal nanoparticles can block charge transfer at metal/semiconductor interfaces involved in applications such as photovoltaic cells, resulting in a significant decrease of the performance of devices fabricated with the surfactant coated metal nanoparticles. The surfactant molecules can also decrease the catalytic efficiency of the metal nanoparticles used for catalyzing chemical reactions because the surfactant coating can block the interaction between precursor molecules and the metal surfaces.
Other techniques such as the combination of templates on wafer substrates and physical deposition (e.g., thermal evaporation, electron-beam evaporation, sputtering, atomic-layer deposition) can avoid the use of surfactant molecules to produce metal nanostructures with various nonspherical shapes defined by the templates. For example, “nanosphere lithography” provides a versatile approach to fabricate metal nanoparticles with a number of morphologies (e.g., triangular plates, rings, overlaps of double triangles, and chains of triangles). However, the preparation of nanosphere templates and the use of vacuum deposition tools in their fabrication can be a complicated and inefficient and costly process.